The present invention relates to the manufacture of semiconductor integrated circuits (ICs) and more particularly to a method for eliminating development related defects referred as polymer blobs in photoresist masks at the end of the photolithography process.
In the manufacture of semiconductor integrated circuits such as Dynamic Random Access Memory (DRAM) chips, polysilicon borderless contacts, referred to hereinbelow as the CB contacts, are extensively used to connect devices to the first level of metallurgy (M0), for instance, to interconnect the source regions and the gate conductors of the Insulated Gate Field Effect Transistors (IGFETs). In conventional DRAM chips, each elementary memory cell is comprised of an IGFET and its associated capacitor that is formed in a deep trench.
The essential steps of a conventional CB contact hole formation process will be briefly described by reference to FIGS. 1A-1D. After these steps have been completed, the CB contact holes are formed and then filled with a conductive material to create the so-called CB contacts.
FIG. 1A schematically illustrates a state-of-the-art semiconductor structure 10 which is part of a wafer at the initial stage of the CB contact hole formation process. Structure 10 comprises a silicon substrate 11 with diffused regions formed therein and a plurality of gate conductor stacks 12 formed thereon. A gate conductor stack consists of a composite SiO2/doped polysilicon/tungsten silicide structure.
Referring to FIG. 1B, structure 10 is coated with a boro-phospho-silicate-glass (BPSG) layer 13 and a tetra-ethyl-ortho-silicate (TEOS) oxide layer 14 above it. These layers are conformally deposited onto structure 10 by LPCVD as standard. As apparent in FIG. 1B, structure 10 has a substantially planar surface.
Now, CB contact holes are formed, using a common deep UV (DUV) photolithography process. To that end, the wafer is placed in an equipment comprised of a clean track system and a DUV exposure tool allowing a fully clusterized operation. For instance, the clean track system is an ACT8 tool manufactured by TEL (Tokyo Electron Limited), Tokyo, Japan and the DUV exposure tool is a Micrascan 3 scanner manufactured by SVG (Silicon Valley Group), Wilton, Conn., USA.
Turning to FIG. 1C, structure 10 is coated first with a 90 nm thick organic Bottom Anti-Reflective Coating (BARC) layer 15 then with a 625 nm thick layer 16 of a DUV photoresist material. After deposition, the photoresist layer 16 is baked, exposed, baked again, then developed as standard to leave a patterned layer referred to hereinbelow as the CB mask still referenced 16 for the sake of simplicity. The purpose of this CB mask 16 is to define the locations of the CB contacts at the first level of metallurgy (M0).
The BARC material supplied by SHIPLEY USA, Malborough, Mass., USA under reference AR3 900 and DUV photoresists such as KrF M20G supplied by JSR Electronics Co, Yokkaichi, Japan or UV80 supplied by SHIPLEY USA are adequate in all respects. The essential process parameters of the different steps to which the wafers are submitted during the photoresist development process are given below. All these steps are conducted in the ACT8 tool.
1. BARC layer: after coating, bake at 225xc2x0 C. during 60 sec, then cool down to 22xc2x0 C. for 60 sec.
2. Resist layer: after coating, post apply bake (PAB) at 140xc2x0 C. during 90 sec, then cool down to 22xc2x0 C. for 60 sec.
3. Post exposure bake (PEB): bake at 140xc2x0 C. during 90 sec followed by cooling at 22xc2x0 C. for 60 sec.
4. Development: conducted in four sub-steps using surfacted TMAH 0,263N that is dispensed with the H nozzle at 22xc2x0 C.:
a) developer puddle formation with a 50 sec wait;
b) developer refresh (PDD: post development dispense);
c) rinse with 22xc2x0 C. deionized water (DIW); and,
d) dry by spin rotation.
After the CB mask 16 has been defined, the process continues with the etching of layers 13 and 14 at locations not protected by said CB mask 16 to create CB contact holes 17. At this final stage of the CB contact hole formation process, the resulting structure is shown in FIG. 1D. Now the CB contacts are fabricated. A doped polysilicon layer is conformally deposited onto structure 10 to fill CB contact hole 17 in excess. Next, the doped polysilicon is etched in a plasma until the TEOS layer 14 surface is reached, and the etching is continued to produce a recess (CB recess) in the polysilicon fill that will be subsequently filled with metal to produce the desired M0 metal lands for the word lines.
To control the defects or contamination added by the photolithography process itself, it is common to inspect patterned monitor wafers using a defect inspection equipment such as the TENCOR AIT, a tool manufactured by KLA-TENCOR, San Jose, Calif., USA, right after the end of the photolithography process. Total or partial wafer surface can be inspected resulting in a defect density measured by the number of defects/cm2. A map of the defects is generated. The defects can then be viewed using an optical microscope with laser imaging to analyze the size and shape of the defects in an attempt to determine the root cause. Bare silicon monitor wafers patterned with the CB mask 16 are used to control the defect level of the above described CB contact hole formation process.
The step of creating the CB mask 16 in DRAM chips is essential to the whole chip fabrication process, CB contact holes not etched can lead to the rejection of the chip. This step is normally a clean process which leads to a defect free photoresist CB mask 16. More generally, less than 15 defects/wafer in the array area has been an acceptable level in the photolithography process for current technologies so far. Unfortunately, the total defect density at the CB mask level has been increasing with the introduction of a new generation of DUV photoresists in the manufacturing lines for unknown reasons.
Recent advances in high resolution DUV photoresists incorporating ESCAP (Environmental Safe Chemically Amplified Photoresist) chemistries have allowed to extend the life of a number of technologies in DUV photolithography beyond 0.20 xcexcm. A side effect of this improved resolution for certain photoresists is the appearance of defects of a new kind that can be widely found in several high resolution DUW photoresists that are commercialized by different vendors on the market to date. These defects, known under the name of xe2x80x9cpolymer blob defectsxe2x80x9d because they are xe2x80x9cblobxe2x80x9d shaped, are seen right after development and can also be qualified as post development residues. Most of the time, they are seen in large unexposed parts of the photoresist layer in the xe2x80x9csupport/kerfxe2x80x9d area but they are also present in the xe2x80x9carrayxe2x80x9d area. If we still consider the CB contact hole formation process described above, the blobs can be redeposited over openings of the CB mask 16 preventing the contact hole formation during the etch step. Blobs are very critical defects as they have a real impact on test yields. The big concern for photoresist users and manufacturers is that as DUV photoresist systems evolve to even higher resolution, polymer blobs will soon become a major yield detractor.
The polymer blobs can vary in size from approximately 1 xcexcm (referred to as small blobs) up to 20 xcexcm or even more (referred to as large blobs). Typical small and large blobs are shown in FIGS. 2A, 2B and 2C respectively. As apparent in FIG. 2A, the small blob located at the center of the photograph covers two CB contact holes and there are some polymer residues over the surrounding CB contact holes. FIGS. 2B and 2C show a typical large blob in the xe2x80x9carrayxe2x80x9d and xe2x80x9csupport/kerfxe2x80x9d areas respectively. Large blobs often have a donut-like shape with an inside circle. A large polymer blob is able to cover a great number of CB contact holes and in that regards, can be considered as a manufacturing yield killer. SEM analysis shows a 10 nm thick circular structure surrounded by little spots. Chemical analysis have shown the presence of traces of metals such as Ca, Na, K and Mg. Blobs are easily seen with a simple optical microscope under dark field during post-development inspection. They exhibit a definite signature in that they form clusters which look like water trails or shiny stars. Although they were originally seen, by hundreds, in the xe2x80x9csupport/kerfxe2x80x9d areas of the CB mask 16, typical defect density in the xe2x80x9carrayxe2x80x9d area is approximately 3.5-4 blobs/cm2 but can go up to 6 defects/cm2 (i.e. 500 defects/wafer). FIG. 3 shows a map of the total defects for an inspected wafer. By total defects, we consider non-blob related and blob defects.
FIG. 4 shows the average number of total defects for three different resists labeled A, B and C that were used at the CB mask level of the conventional CB hole formation process described above by reference to FIGS. 1A-1D. As apparent in FIG. 4, the blob density is equal to about 4 defects/cm2 for resist A. For all experiments, the wafers were inspected with the TENCOR AIT. A complete review was then done which allowed to separate non-blob related defects from small and large blob defects. Polymer blobs are a major concern at the CB contact hole formation level but they have also been identified at other masking levels, particularly at contact-like openings and appear to be very dependent on pattern density. Note that, they also appear on line-space patterns but in a much smaller extent.
Extensive works have been done so far to eliminate polymer blob defects by incorporating process-specific solutions. On the other hand, photoresist vendors are working hard on new formulations. Proposed fixes, which are usual known fixes in the photolithography process, include lowering PAB and PEB temperatures, optimizing development or DIW rinse cycle, adding delays between exposure and PEB, increasing exhaust in the developer module, slightly increasing developer temperature, etc. All these fixes reduce the amount of defects, some just a few, others a little bit more, and when combined together (at the cost of an obvious increased process complexity), the defect level decreases dramatically to 1 blob/cm2, i.e. an 75% improvement, but none combination of fixes has been identified so far that would totally eliminate the blob defects. An acceptable fix would be to have a 95% defect elimination which would represent a defect density of about 0.2 blob/cm2, although the goal would be to reach the 100% total elimination.
It is therefore a primary object of the present invention to provide an efficient method to totally eliminate development related defects called polymer blobs in patterned photoresist masks.
It is still another object of the present invention to provide an efficient method to totally eliminate development related defects called polymer blobs in patterned photoresist masks that improves manufacturing yields by a significant decrease of the chip rejection rate.
It is another object of the present invention to provide an efficient method to totally eliminate development related defects called polymer blobs in patterned photoresist masks at the CB contact hole formation level in DRAM chips.
The accomplishment of these objects and other related objects is achieved by the two methods of the present invention which in substance rely on a heat treatment of the wafer during or after the development step. In the first method, after the standard development step has been carried out, the wafer is heated (e.g. at 140xc2x0 C.) by performing a post development bake (PDB) without cooling that is immediately followed by an extra rinse with deionized water (DIW) at room temperature (22xc2x0 C.). In the second method, the wafer is rinsed with hot deionized water (e.g. at 60xc2x0 C.) either in an extra rinse step after the development has been performed or during the development step as a replacement of the standard 22xc2x0 C. DIW rinse.
The above methods are applicable to any photolithography process whatever the type (MUV/DUV) of masks used and the type of photoresists and lead to a significant improvement of the blob defect density in the patterned photoresist masks.